As vehicles accumulate mileage, the catalytic converter deteriorates. The deterioration results in a less efficient catalyst. To monitor catalytic converter deterioration, the fuel injection system uses two heated oxygen sensors. One sensor is upstream of the catalytic converter; one is downstream of the converter. The Powertrain Control Module (PCM) compares the readings from the sensors to calculate the catalytic converter oxygen storage capacity and converter efficiency. Also, the PCM uses the upstream heated oxygen sensor input when adjusting injector pulse width.
When the catalytic converter efficiency drops below emission standards, the PCM stores a Diagnostic Trouble Code (DTC) and illuminates the Malfunction Indicator Lamp (MIL).
The Heated Oxygen Sensors (HO2S) produce a constant 2.5 volts on NGC (4-cylinder) vehicles, depending upon the oxygen content of the exhaust gas. When a large amount of oxygen is present (caused by a lean air/fuel mixture-often caused by misfire and exhaust leaks), the sensors produce a low voltage. When there is a lesser amount of oxygen present (caused by a rich air/fuel mixture-often caused by internal engine problems) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.
The oxygen sensors are equipped with a heating element that keeps the sensors at the proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.
In Closed Loop operation, the PCM monitors the HO2S input (along with other inputs) and adjusts the injector pulse width accordingly. During Open Loop operation, the PCM ignores the HO2S input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.
The NGC Controller has a common ground for the heater in the HO2S. 12 volts is supplied to the heater in the HO2S by the NGC controller. Both the upstream and downstream HO2S for NGC are Pulse Width Modulation (PWM).
- Using a diagnostic scan tool, check for the presence of any Diagnostic Trouble Codes (DTC-s). Record and address these codes as necessary.
- If no codes are present, review the scan tool environmental data. If possible, try to duplicate the conditions under which the DTC set.
- If applicable, actuate the component with the scan tool.
- Monitor the scan tool data relative to this circuit and wiggle test the wiring and connectors.
- Look for the data to change, the actuation to be interrupted, or for the DTC to reset during the wiggle test.
- Refer to any Technical Service Bulletins (TSBs) that may apply.
- Turn the ignition off.
- Visually inspect the related wire harness. Disconnect all the related harness connectors. Look for any chafed, pierced, pinched, partially broken wires and broken, bent, pushed out, or corroded terminals.
- Perform a voltage drop test on the related circuits between the suspected component and the Powertrain Control Module (PCM).
- Inspect and clean all PCM, engine, and chassis grounds that are related to the most current DTC.
- If numerous trouble codes were set, use a schematic and inspect any common ground or supply circuits.
- For intermittent Misfire DTC-s check for restrictions in the Intake and Exhaust system, proper installation of Sensors, vacuum leaks, and binding components that are run by the accessory drive belt.
- Use the scan tool to perform a System Test if one applies to the component.
- A co-pilot, data recorder, and/or lab scope should be used to help diagnose intermittent conditions.